A food processor increased its production capacity and installed a rental compressor to feed the new load. Previous compressors were lubricated type with refrigerated dryers. The new compressor was a 200-hp oil-free variety with heatless desiccant dryer. A 1,060-gal storage tank was installed to work with the compressor, which operated in load/unload mode.
A compressed air assessment was done at the site sometime after — and found some surprising results. In 1.5 years of operation, the compressor had cycled more than 700,000 times! Typically, one million cycles would be seen through a 10 to 15 year life of a compressor. Observation of the compressor showed that the compressor was loading and unloading every four to six seconds.
Examination of the system showed there was a problem with check valves: the air dryer had two valves preventing system pressure from coming back to the compressor pressure sensing transducer, and there was a secondary check valve between the dryer and the receiver.
When the dryer was purging, and the compressor would unload, the dryer purge flow would immediately cause the pressure to crash, requiring the compressor to quickly reload. The compressor was cycling itself to death. These check valves caused the compressor to be blind to the system storage. This is typical of any system with desiccant dryer with internal check valves that is matched with a single compressor.
Flow testing determined that this compressor was using power as it if was fully loaded, yet was putting out only half of its rated capacity.
The solution for this system is to install remote sensing, or to install a check valve protected bypass line that allows system pressure back to the discharge of the air compressor. In this way, the compressor will be able to see true system pressure and the cycle frequency will go back to a normal value. The plant is modifying the system to do just that, and doing some needed repairs to the compressor soon.